Every nozzle used by pressure washing contractors is rated in two ways - pattern and orifice. The pattern is the "shape" the water takes (15 degree, 25 degree, etc.). The orifice is the diameter of the opening, and that is the most important number to understand. If you buy a pressure wash nozzle with an orifice that is too small, you are choking off your power washer. If you buy an orifice too big, your equipment will not deliver the power it was designed for.
There is a link to a Nozzle Chart below. Feel free to print this for your future reference.
First locate the PSI of your machine along the top line. Once you have found that column, run your finger down the column to the APPROXIMATE gallons-per-minute of your pressure washer. Once you have found the closest number, move your finger out to the left-hand column and see the recommended orifice size.
Once you know the proper orifice size for your machine, you can reduce the output pressure simply by increasing the orifice size. To do that, locate the PSI you want to achieve. Once you have found that column, run your finger down to the APPROXIMATE gallons-per-minute of your pressure washer. Once you have found that, simply move your finger to the left hand column and see what orifice size will give you the pressure output you want.